Because human bile contains a lot of secretory IgA, it has been suspected that the human liver, like rat liver, transfers polymeric IgA from plasma to bile. Hence, a rich source of polymeric IgA might enter the general circulation of man. We examined human thoracic duct lymph, portal vein blood and aortic blood for content and molecular size of IgA. None of the fluids was found to have either a higher total concentration of IgA or a higher proportion of polymeric IgA than that found in peripheral venous blood. It is possible that hepatic clearance of plasma IgG does not occur in man to the extent that it does in the rat, and a relatively larger proportion of human biliary IgA might originate from synthesis in hepatobiliary tissues.